The invention concerns an internal combustion engine, in particular a stationary gas engine, comprising a combustion chamber to which an engine fuel can be fed from a first engine fuel source by a combustion chamber conduit, and a prechamber to which a scavenging gas can be fed by a scavenging gas conduit. A scavenging gas mixer received an engine fuel which can be fed by an engine fuel conduit from the first engine fuel source or from a second engine fuel source, and a synthesis gas which can be fed by a synthesis gas conduit to be mixed. A mixer outlet opens into the scavenging gas conduit, and the synthesis gas can be produced by a reformer to which a combustion fuel can be fed by way of a reformer feed conduit from a combustion fuel source and the reformer outlet of which opens into the synthesis gas conduit.
In internal combustion engines which are operated on the basis of the Otto cycle, ignition of a fuel-air mixture is effected in the combustion chamber by ignition devices. Mixture ignition is generally initiated by a spark flash-over at the electrodes of a spark plug. Alternatively, it is also known to use a laser spark plug as the ignition device, in which case the required ignition energy is introduced into the combustion chamber in the form of laser light. Particularly in gas engines in which a fuel gas-air mixture is ignited, the lean burn concept is used in relation to larger combustion chamber volumes. That means that there is a relatively great air excess, whereby at maximum power density and at the same time with a high level of efficiency of the engine, pollutant emission and the thermal loading on the components is minimized. Ignition and combustion of very lean fuel-air mixtures represents in that case a considerable challenge for development and operation of modern high-power gas engines.
As from a certain structural size of the gas engines (generally approximately above six liters capacity), it is necessary to use ignition boosters in order to pass through the correspondingly long flame paths in the combustion chambers of the cylinders in the shortest possible time. Prechambers usually serve as such ignition boosters, and the fuel-air mixture which is highly compressed at the end of the compression stroke is ignited in a relatively small secondary chamber separated from the main combustion chamber of the cylinder. In that case, a main combustion chamber is defined by the working piston, the cylinder liner, and the cylinder head surface, and the secondary chamber (the prechamber) is connected to the main combustion chamber by one or more flow transfer bores. Frequently such prechambers are scavenged or filled with engine fuel gas during the charge change phase to enrich the fuel-air mixture and thus improve the flame and combustion properties. For that purpose, a small amount of engine fuel gas is branched from the engine fuel gas feed to the main combustion chamber and introduced into the prechamber by way of a suitable feed device provided with a non-return valve. That amount of engine fuel gas scavenges the prechamber during the charge change and is therefore often referred to as a scavenging gas.
During the compression phase, the very lean fuel-air mixture of the main combustion chamber flows through the flow transfer bores into the prechamber and is there mixed with the scavenging gas. The ratio of engine fuel to air in the mixture is specified in the form of the air excess index λ. An air excess index of λ=1 means in that respect that the amount of air present in the mixture precisely corresponds to that amount required to permit complete combustion of the amount of engine fuel. In such a case, combustion takes place stoichiometrically. Under full load, large gas engines are usually operated lean with a λ of between about 1.9 and 2.0, that is to say the amount of air in the mixture approximately corresponds to double the stoichiometric amount of air. Scavenging of the prechamber with engine fuel gas, after mixing with the engine fuel gas-air mixture from the main combustion chamber, gives a mean λ in the prechamber of between about 0.8 and 0.9. That affords optimum flame production conditions, and by virtue of the energy density intensive ignition flares which issue into the main combustion chamber and which lead to the fuel-air mixture in the main combustion chamber rapidly burning through. With such λ values, combustion however takes place at a maximum temperature level so that the wall temperatures in the prechamber region are also correspondingly high. That results on the one hand in a correspondingly high thermal loading on the prechamber and the components arranged therein (for example spark plug, valves), and on the other hand in unwantedly high nitrogen oxide emissions.
With an increasing rise in the engine power output and by virtue of the measures for the increase, the level of efficiency soot formation also increasingly occurs in the prechamber. The soot content resulting therefrom in the engine exhaust gas leads to impairment of heat transfer in the heat recovery steam generator and problems in certain applications of gas engines, for example for CO2 fertilization of greenhouses.
A possible way of avoiding soot formation involves leaning off the fuel-air mixture in the prechamber and oxidizing the free carbon by a slight oxygen excess. In that case, however, other problems arise which are related to the fact that excess oxygen at the very high combustion temperatures just above λ=1 can lead to hot corrosion at critical locations in the prechamber, in particular at the flow transfer bores and at the spark plug electrodes.
It is also known from the state of the art for the scavenging gas to be fed to a prechamber to be enriched with suitable gases in order to increase the ignition quality of the scavenging gas in lean-burn operation of the internal combustion engine. Thus, U.S. Pat. No. 6,739,289 B2 discloses a method of enriching a prechamber scavenging gas with hydrogen. In that case, the engine fuel for the prechamber is passed through a reformer to enrich the engine fuel with hydrogen. Known thermochemical reactors such as for example steam reformers can be used as the reformer. It is to be noted that the known apparatuses and methods have the disadvantage that a direct feed of the synthesis gas produced by the reformer to the prechambers of the internal combustion engine results in a reduced service life for the prechambers and the components arranged therein.